1. Field of the Invention
This invention relates generally to electrophotography and, more particularly, this invention relates to printing element compositions and methods.
2. Description of Related Art
Electrophotography is an imaging process that typically involves the formation of an electrostatic image on a somewhat electrically conductive, solvent impervious photoconductive medium. It is known that zinc oxide can be employed in making photoconductive layers on a paper substrate and that photographic copies can be prepared from papers coated with such photoconductive compositions.
Generally, electrophotographic paper is prepared by applying a coating comprising a mixture of zinc oxide and a suitable binder to a conductive paper substrate. Prior to exposure to light, the zinc oxide layer is given a negative electrostatic charge by means of ion transfer from a corona discharge. When the charged paper is exposed to an optical image, electrical conductivity is produced in the areas where light strikes the coating, and the charge dissipates through the conductive sheet. The print area not exposed to light retains a latent image in the form of an electrostatic charge. The exposed coating is developed by attracting toner (positively charged pigment particles dispersed in a low melting-point resin) to the negatively charged print areas. If a solid toner is used, heat may be used to fuse the toner to the zinc oxide coating. If a liquid toner is used, heat is not required to fuse the toner to the zinc oxide but may be used to evaporate the solvent.
The above described process for the preparation of a print may be used for the preparation of an offset print or offset master for use in offset printing wherein the "master" serves as an intermediate in the process of making an image from an original. In offset printing, the master is contacted with a conversion solution which makes the background, i.e. the zinc oxide, hydrophilic while the imaged areas remain hydrophobic. The offset rollers then apply ink and water to the converted master. Ink is transferred to an offset cylinder, commonly referred to as a "rubber blanket". The ink is subsequently transferred from the blanket to paper, thereby effecting the offset printing of the image.
Unsensitized zinc oxide normally has a spectral . response in the ultraviolet region of the spectrum, at about 386 nanometers (nm). Photoconductive layers consisting of unsensitized zinc oxide are slow to respond to light and therefore require considerable exposure time to form latent electrostatic images. To obviate these problems, those in the art have sought to sensitize the photoconductive materials, such as the zrnc oxide used in photoconductive coatings, by the addition of sensitizing dyestuffs which are capable of absorbing the radiant energy of the wavelengths to which the photoconductive material is otherwise substantially insensitive and which are further capable of transferring the absorbed energy to the photoconductive material. For an electrophotographic process, zinc oxide is generally sensitized to be responsive in the visible region of the spectrum at a wavelength between about 500-620 nanometers, by the addition of low concentrations of sensitizing dyes.
Cyanine dyes are among the dyes that have been found to be most useful for spectral sensitization of zinc oxide. Cyanine dye sensitization has been disclosed in U.S. Pat. No. 3,619,154 (Nov. 9, 1971) to G. A. Cavagna et al. Zinc oxide electrophotographic coatings containing cyanine sensitizers and multi-component binders are disclosed in U.S. Pat. No. 3,682,630 (Aug. 8, 1972) to W. C. Park et al.
Offset masters are commonly made by exposure of a photosensitive sheet to visible light. The use of lasers for the making of masters, however, allows master radiation input information to be easily translated to a digital form. Digital input facilitates and permits the efficient use of computers for the storage of such master input information.
The electrical properties of zinc oxide coatings that affect print quality include charge acceptance, dark decay rate, light decay rate, and residual voltage. Coatings with high charge acceptance attract more toner and consequently produce denser prints. A fast light decay rate and a low residual voltage combine to increase the speed of photoresponse and give clean prints, free of background. Higher production rates can be achieved through the use of coatings which have a comparatively high speed of photoresponse. Dark decay rate is important if it is excessively high. Print density is a function of the voltage on the coating at the time it passes through the toner and the coatings that have lost a large portion of the original charge because of a high dark decay rate have poor print density regardless of initial charge acceptance voltage.